1. Field of the Invention (Technical Field)
The present invention relates to sensors and methods for non-destructive evaluation (NDE) of structures such as bridges, aircraft, building supports, etc.
2. Description of Related Art
Structural health monitoring (SHM) is a growing concern of the engineering community. This need is even more intense in the case of aging aerospace and civil structures, many of which are operating well beyond their initial design lives. An unavoidable by-product of a metallic structure's use is the development of crack, corrosion, and other flaws (“faults”) that degrade the structure over time. Significant cost barriers to replace these structures have created an aging infrastructure. These economic realities, coupled with an explosion in infrastructure development and unexpected phenomena that have accelerated decay, have placed even greater demands for structural health monitoring. In addition to the growth in infrastructure, the effort and frequency of maintenance and surveillance also increases with age. As a result, the costs to manage our aging infrastructure are rising at an unexpected rate. For example, aircraft maintenance and repairs represent about a quarter of a commercial fleet's operating costs.
Therefore, it is imperative that methods and apparatuses be developed which can quickly and easily assess the integrity of the structure. The application of distributed sensor systems can reduce these costs by allowing condition-based maintenance practices to be substituted for the current time-based maintenance approach. In addition, innovative deployment methods must be devised to overcome a myriad of inspection impediments stemming from accessibility limitations, complex geometries, and the location and depth of hidden damage.
Prevention of unexpected flaw growth and structural failure could be improved if remotely-accessible, on-board health monitoring systems are used to continuously assess structural integrity. Such systems would be able to operate in the field for extended periods and detect incipient damage before catastrophic failures occurs. With detection capabilities, failure signatures and the corresponding driving factors can be extracted from the data. As a result, condition-based maintenance practices could be substituted for the current time-based maintenance approach.
A “Smart Structure” is one which is sufficiently instrumented so that the data can be synthesized to form an accurate real-time picture of the state of the structure in all its critical aspects. By positioning sensors around the critical regions of the structure (i.e., locations where fatigue cracks are most likely) it is possible to establish an in-situ sensor network for continuous, real-time structural health monitoring.
A useful aspect of a “Smart Structure” is the ability to provide subsurface fault detection, in particular for cracks. Such crack detection requires a sensor with penetrating (strong EMF, or other Non-Destructive Evaluation (NDE) methodology) capability, yet able to be placed on a thin Process Control Device (PCD) (low profile) for mounting. Furthermore, it must also be sensitive enough to detect small cracks through: (1) non-conducting lift-off impediments such as composite, plastic or other coatings, and (2) conducting layers such as steel or aluminum so that sub-layer cracks can be detected.
The present invention provides such a sensor, providing deep penetration with a strong enough magnetic field to interrogate thick, mutli-layer, conductive structures. It can also overcome the “lift-off” effects of nonconductive layers/coatings. Such lift-off effects lower the strength of the eddy current (EC) field in the area of interest, and so the coil design in this sensor, along with its mode of operation, is a critical aspect of the sensor's unique capabilities.
Other advantages of on-board distributed sensor systems are that they can eliminate costly, and potentially damaging, disassembly, improve sensitivity by producing optimum placement of sensors with minimized human factors concerns in data acquisition, and decrease maintenance costs by eliminating more time-consuming manual inspections.
Through the use of the sensor of the invention it is possible to quickly, routinely, and remotely monitor the integrity of a structure in service. It provides a reliable structural health monitoring system that can automatically process data, assess structural condition, and signal the need for human intervention. This sensor is able to detect incipient damage before catastrophic failures occur.
The replacement of present-day manual inspections with automatic health monitoring would substantially reduce the associated life-cycle costs. Another important item to note is that the ease of monitoring an entire network of distributed sensors means that structural health assessments can occur more often, allowing operators to be even more vigilant with respect to flaw onset. In addition, corrective repairs initiated by early detection of structural damage are more cost effective since they reduce the need for subsequent major repairs.
Objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.
The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.